1. Field of the Invention
The present invention relates generally to the control of a centrifugal liquid chiller, and more specifically to the use of a hot gas bypass valve or another orifice in a centrifugal liquid chiller to minimize surge.
2. Description of the Related Art
As generally known, surge is an unstable condition that may occur when compressors, such as centrifugal compressors, are operated at light loads and highpressure ratios. It is a transient phenomenon characterized by high frequency oscillations in fluid pressure and flow, and, in some cases, a complete flow reversal through the compressor. Surge, if uncontrolled, causes excessive vibrations that may result in permanent damage to the compressor. Further, surge may cause excessive electrical power consumption if the drive device is an electric motor.
It is generally known that a hot gas bypass (HGBP) flow helps avoid surge during low-load or partial-load conditions. As the cooling load decreases, the requirement for hot gas bypass flow increases. The amount of hot gas bypass flow at a certain load condition is dependent on a number of parameters and is controlled by a HGBP valve.
A HGBP valve control in the prior art provides for an analog electronic circuit that outputs a DC voltage signal that is proportional to the required opening of the valve. The prior art system, however, requires manual calibration at two different chiller operating points at which the compressor just begins to surge. As a consequence, a good deal of time is spent performing the calibration with the assistance of a service technician. Further, variation of coolant flow, which is necessary for many applications, requires repeated calibration.
Another disadvantage of the prior art is that it makes the false assumption that the xe2x80x9csurge boundary,xe2x80x9d which defines the conditions under which the compressor would surge as a function of certain parameters, is a straight line. Instead, it is often characterized by a curve that may deviate significantly from a straight line at various parameters. This inaccuracy of the prior art may cause the HGBP valve to open prematurely, or it may allow the unit to surge unnecessarily at the operating conditions.
Thus, it is desirable to provide an automatic control system for the HGBP valve or other flow control devices to provide optimal control that is responsive to the characteristic of a given centrifugal chiller system.
This summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show several embodiments of the invention and, together with the description, explain the principles of the invention.
Methods and systems consistent with this invention control a hot gas bypass valve in a refrigeration system including a centrifugal compressor, a condenser, an evaporator, and a hot gas bypass line between the compressor and the evaporator. Such methods and systems continuously sense for a surge condition during operation of the refrigeration system, indicate a surge condition when the refrigeration system is operating under surge conditions, and open at least partially the hot gas bypass valve in response to the sensed surge condition to return the refrigeration system to operating under non-surge conditions.
Methods and systems consistent with this invention control a hot gas bypass valve in a refrigeration system including a centrifugal compressor, a condenser, an evaporator, and a hot gas bypass line between the compressor and the evaporator. Such methods and systems sense a present head parameter representative of the present head of the compressor, sense a present load parameter representative of the present load, and control the hot gas bypass valve so as to avoid surging in the compressor in response to the present head parameter, the present load parameter, and stored head and load parameters.